This invention relates to techniques for recording digital information on magneto-optical storage media.
The exploding demand for computer memory has propelled current research in memory systems in the direction of magneto-optical (M-O) technology. The M-O medium, typically in the form of a disk, comprises a thin magnetic layer covered with a relatively thick transparent coating. Digital information is stored in the M-O medium by locally magnetized regions or domains in the magnetic layer of one polarity or another corresponding to "1's" and "0's". While the information is thus retained magnetically in a manner analogous to conventional magnetic media, the writing and reading processes usually involve laser beams. M-O writing is thermally assisted. A pulsed laser beam is focused through the transparent overcoat onto the surface of the magnetic layer. The coercivity of the magnetic media exposed to the beam is temporarily lowered by the heat induced by the laser pulse, enabling the local orientation of the magnetic domains to be redirected by means of a magnetic field. Reading is accomplished through Kerr or Faraday rotation of the angle of polarization of a low power (nonheating) incident laser beam ordinarily supplied by the same laser used in writing. Depending on the local orientation of the magnetic media, the polarization angle of the reflected beam rotates slightly clockwise or counterclockwise. This shift in the polarization angle determines whether the cell contains a "1" or a "0". Facilitating the overwriting of memory cells in M-O media, thereby changing the contents of the cells, is of major interest because it allows M-O disks to replace lower capacity, less rugged magnetic media systems in a variety of commercially important applications.
The most common approach for writing on an M-O disk requires a two pass operation. First, the area of the disk to be written is erased in order to bring the magnetic domains in that area into a known uniform condition. To perform this erase operation, an external magnetic field, provided by a magnetic coil, a permanent magnet or even another layer in the disk, is applied to a region of the disk on the first pass of the write operation. Once the erase cycle has oriented the magnetic dipoles in a known direction (defined as "0"), the writing portion of the operation begins. To write the disk, the external magnetic field is reversed and selected domains are thermal magnetically written as "1" bits, leaving the "0" bits unchanged. This scheme requires a minimum of two passes to write data, i.e., an erase cycle and a write cycle, and so reduces the performance of the M-O system.
Recently an overwrite scheme has been proposed which employs the self-demagnetization field of the M-O media itself to change the polarization of a desired memory cell such that no external magnetic field is required. See Shieh and Kryder, "Operating Margins for Magneto Optic Recording Materials with Direct Overwrite Capability", IEEE Transactions on Magnetics, Volume 23, No. 1, pages 171-173 (1987), incorporated by reference herein. (A copy of a preprint of the foregoing article accompanies this application in the Information Disclosure Statement filed simultaneously herewith.) The basic principle, as described in the Shieh et al article, is described briefly herebelow.
Self demagnetization is the tendency for the orientation of the dipoles in a magnetic material to align with opposite orientation so as to minimize their energy. The lowest energy configuration for a system of dipoles is with alternating polarities. Within a magnetic material, there are two kinds of pertinent magnetic forces: the long-range magnetic force and the short-range exchange force. Exchange forces between dipoles tend to keep adjacent dipoles aligned while the long range forces tend to impose on any one dipole a force proportional to the orientation of the rest of the dipoles in the material. The region of aligned dipoles is a domain and the demarcation between domains is called the Bloch wall. It is this balancing of the short and long range forces that keeps the domain from growing until it encompasses the entire material or from shrinking until the orientation of all of the dipoles is randomized. The process of creating a domain begins with heating a local region of the magnetic material by a laser pulse of a certain energy level. The dipoles in the region change their orientation because the coercivity of the region is lowered by the heating process. The self-demagnetization force can then force the dipoles in the region to align in the opposite direction thus creating a domain of opposite polarity.
To erase a domain, a shorter duration pulse is directed toward an existing domain. The erasure pulse should be approximately centered with respect to the original domain. There is again a decrease in coercivity which causes the central portion of the domain to flip with respect to the rest of the domain. Once this occurs, the domain comes unstable and the exchange energy causes the domain wall to move outward. In a short time, this realignment moves through the domain, acting as an erasure.
Thus, pulses of different duration can be used to either erase or write regions of an M-O disk without the need for an external magnetic field.
The Shieh and Kryder article states that when the foregoing system is coupled with a read-before-write scheme, it is possible to achieve direct overwrite. In their proposal, two laser beams are used. The first beam reads information on a previously written track. The second laser beam does the erasing or writing. Let us assume that the M-O disk has been initialized to all "0's". The first beam, which can be referred to as a "scout", reads the track to see whether a particular bit is already a "1" or a "0". If the scout beam determines that the bit is a "1" and the new data to be written is also a "1", the second laser beam is not activated. Similarly, if the scout beam reads a "0" and the second beam was to write a "0", the second beam is not activated. If however, the scout beam detects a "0" when a "1" needs to be written, a write pulse is applied via the second laser beam to align the dipoles and create a domain. If a "1" is present and a "0" needs to be written, the second laser beam applies an erase pulse to annihilate the existing domain.
In addition to providing one of the inputs to the logic which determines whether to activate the second laser beam, the first or scout beam also provides timing information enabling the second beam to be directed accurately at the memory cell so that the second beam is approximately centered on an existing domain in the case of an erasure. However, in order for the timing information to be valid, the distance between the scout and write/erase beams has to be kept constant with high precision. Due to thermal distortion and relaxation, this is in practice difficult to achieve.